The multimode fiber—the initially introduced optical fiber structure expected to dramatically change communications—did not fulfill expectations. The laser-yielded pulse stream to be modulated by signal information, single-mode as introduced at the transmitter, was no longer single-mode as detected at the receiver. Collisions with defects in the glass fiber caused mode conversion, ultimately yielding pulses constituted of photons of a broad spectrum of higher-order modes, extending even beyond those which could be guided by the fiber structure (to “lossy” modes—to modes soon lost by radiation). Consequential mode-dispersion resulted in pulse spreading—with lower order modes at the leading edge of the pulse and higher order at the trailing edge. This restricted the pulse repetition rate of the injected train to the maximum rate with sufficient retained pulse separation to enable detection of now-broadened pulses upon arrival at the detector. This led to an extensive effort directed to limiting pulse broadening. (Added loss, due to radiation of lossy modes was not initially a focal point.)
The result of the effort is represented by the “perturbed” fiber of U.S. Pat. No. 3,966,446, issued Jun. 29, 1976. The patentee, S. E. Miller, pioneered in an approach born of desperation. Expedient means for reducing defect count in the glass, and thereby limiting mode conversion, had not been found—and this is true to the present day. The Miller approach was to, instead, confront the traveling pulse with a sufficiently increased number of mode conversion centers so that each photon arriving at the detector would have spent a near-equal amount of time as every permitted mode—in order to “average” photon transit times—thereby lessening variation in photon arrival times, and restricting pulse spreading. “Perturbations” in index-of-refraction, corresponding with deliberate changes in glass composition, introduced during formation of the fiber preform and carried over into the drawn fiber, would serve as mode coupling centers (as conversion centers) to bring about extensive ongoing mode conversion. To implement the approach, Dr. Miller introduced a new fiber fabrication method—one permitting the abrupt index changes which, until then, had been anathema to the fiber fabricator.
H. M. Presby, in U.S. Pat. No. 4,038,062, issued Jul. 26, 1977, used changes in size/shape—“geometric perturbations” rather than Miller's “compositional perturbations”—introduced in a process offering good perturbation uniformity. Rather than transferring the perturbations from the preform to the fiber, the Presby process utilizes a pulsating heat source which, acting on the already-drawn fiber, yields perturbations of near-constancy in size and spacing.
To large extent, development effort directed to more general use of mode-mixing lost momentum with introduction of single-mode fiber. A form of mode-mixing, still permitted in single-mode fiber, however, is addressed in a commercial process in common use. In that process, dispersion between polarization modes is lessened by repeatedly rotating the fiber undergoing drawing through a ninety degree angle.
There was, however, a continuing interest in multimode fiber, for use, e.g., in local area networks, and, withal, some continuing effort on perturbed fiber. Slackened effort, at this time, is ascribed to the added loss due to eventual coupling to, and radiation loss of, unguided modes (of modes of too high order to be supported by the fiber). That loss penalty has, in general, been too high a price to pay for increased pulse repetition rate.